Then I developed another application that used the same extension. When launching the second application I receive CustomMessage "multiply defined" error. So I did some testing and research and found that Persistable is likely the cause and found that CustomMessage implements Persistable.

I read several post about not using Persistable in libraries, and one reference saying it is OK when used in extensions. I tried refractoing CustomMessage so that is has a unique name, but then end up with a similar error: ReadableListImpl multiplydefined.